Brakes in automobiles or other motor vehicles generally include a friction material and a counter-friction material such as a rotor or drum. The rotor or drum is generally steel or other metal material having a surface thereon adapted for contact with a friction material. The brake pad which is usually a steel or other substrate having a friction material thereon, the surface of the friction material being adapted for contact with the friction surface of the rotor or drum.
One problem with metal rotors and drums is that they are susceptible to oxidation or corrosion. If the brakes are used often, this problem is not too severe as the friction between the rotor or drum and the brake pad is usually sufficient to remove any thin film of oxidation or corrosion. However, if the brakes are not used for a substantial period of time, such as on a new vehicle kept in a seller's lot, the resulting buildup of oxidation or corrosion on the rotor's or drum's friction surface can adversely affect the performance of the brake. The corrosion can become so severe that it results in pitting or scarring of the rotor or drum surface, which can render the rotor unsuitable for achieving the desired braking performance. In some instances, the corrosion can cause the rotor or drum to bond to the friction pad, which can result in cohesive failure of the friction pad or separation of the pad from its substrate.
One solution to this problem is to use rotors that have been coated or otherwise surface treated to inhibit corrosion of the rotor. Such treatments include oil or grease or the like, electrodeposition coatings such as cathodic electrodeposition of amine-salted epoxy resins curable with isocyanate crosslinkers, other curable coatings such as melamine-cured OH-functional acrylic coatings, thermoplastic coatings such as high-molecular weight acrylic or polyurethane latexes or oil-based alkyd coatings, metal oxide coatings where a thin film of oxidation is allowed to form without pitting, and inorganic coatings such as zinc particles dispersed in a potassium silicate solution.
A problem with treated rotors is that although they are resistant to corrosion, the treated surface often does not provide adequate braking performance, especially when matched with a friction material on the brake pad that is itself designed for use with the underlying metal out of which the rotor is made.
It has been proposed to provide an abrasive coating on the brake pad that would have the effect of removing a protective varnish coating on the rotor during the initial break-in period of the brakes (e.g, during the first 200 brake applications). For example, it has been proposed to use a coating of abrasive carborundum particles on the brake pad for this purpose. Such a coating, however, produces an undesired increase in the new or `green` friction of the brakes, resulting in unpredictable, noisy, or `sticky` braking action. Also, with such coatings containing high levels of aggressive abrasive particles, enough particles can embed in the surface of the pad so as to prolong the undesirable performance characteristics.
It is thus an object of this invention to provide a brake pad that can remove a corrosion-resistant coating from a rotor during the initial break-in period of the brakes. It is a further object of the invention to provide a brake pad that maintains good braking performance both during and after the break-in period.